Intervertebral implant

ABSTRACT

An intervertebral implant having a three-dimensional body ( 10 ) and a securing plate ( 1 ). The three-dimensional body ( 10 ) includes an upper side ( 1 ) and an underside ( 2 ) which are suitable for abutting the end plates of two adjacent vertebral bodies, a left side surface ( 3 ) and a right side surface ( 4 ), a front surface ( 5 ) and a rear surface ( 6 ), a horizontal middle plane ( 7 ) between the upper side ( 1 ) and the underside ( 2 ), and a vertical middle plane ( 12 ) extending from the front surface ( 5 ) to the rear surface ( 6 ). The three-dimensional body further includes a plurality of boreholes ( 9   a ) passing through the body ( 10 ), which are suitable for accommodating longitudinal fixation elements ( 20 ). The intervertebral implant also includes a front plate ( 8 ) displaceably disposed as an insert with the front side ( 5 ) of the three-dimensional body, the front plate ( 8 ) having a plurality of boreholes ( 9 ) in which the longitudinal fixation elements ( 20 ) can be anchored, and whose openings overlap with the openings of the boreholes of the three-dimensional body ( 10 ). A securing plate can be fastened essentially parallel to the front plate ( 8 ) at the three-dimensional body ( 10 ) in such a manner that the boreholes of the front plate ( 9 ) are covered at least partly by the securing plate ( 18 ). By virtue of the configuration of the intervertebral implant, a rigid, firm connection between the intervertebral implant and the longitudinal fixation elements used to fasten it, is possible.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/199,599, filed Aug. 8, 2005, which is a continuation of InternationalPatent Application No. PCT/CH2003/000089, filed Feb. 6, 2003, the entirecontents of which are expressly incorporated herein by referencethereto.

TECHNICAL FIELD

The present invention relates generally to intervertebral implants.

BACKGROUND OF THE INVENTION

GB-A-2 207 607 discloses an intervertebral implant, which has ahorseshoe-shaped configuration with a plurality of cylindrical holes.The holes are smooth on the inside and only have a stop for the heads ofthe bone screws, which are to be introduced therein. A disadvantage ofthis arrangement is that the fastening screws, introduced therein, canbe anchored only with their shaft in the bone. This does not result in arigid connection with the horseshoe-shaped intervertebral implant. Whenthe anchoring of the screw shaft in the bone is weakened, theintervertebral implant becomes movable with respect to the screw and thebone screws tend to migrate, endangering the blood vessels. Moreover,the loosening of the intervertebral implant can lead to apseudoarthrosis.

U.S. Patent Publication US-A 2000/0010511 (Michelson) discloses anintervertebral implant, which, at its front surface, has two boreholeswith an internal thread, into which bone screws with a threaded head canbe introduced. A disadvantage of this implant is that the bone screwscan become loose and are not secured against being screwed out orfalling out. A further disadvantage is that the bone screws are fastenedcompletely to the implant body itself and that therefore the latterexperiences a relatively large stress.

Screws which emerge at the anterior or anterolateral edge of thevertebral body because of loosening run the risk of injuring mainvessels such as the aorta and Vena calva, as well as supply vessels suchas lumbar arteries and veins. Injury to these main vessels may result ininternal bleeding possibly causing death within a very short time.Loosening of screws is more likely when they are not mounted angularlyfirmly.

SUMMARY OF THE INVENTION

The present invention is to provide a remedy for the above-discusseddisadvantages. The present invention is directed to an intervertebralimplant which can enter into a permanent, rigid connection with bonefixation means, so that, even if the bone structure is weakened, thereis no loosening between the intervertebral implant and the bone fixationmeans. Moreover, over a separately constructed front plate, there istension chording for the bone fixation elements, so that the implantbody experiences less stress, that is, superimposed tensions. Moreover,a securing plate enables all bone fixation elements to be securedsimultaneously.

The present invention accomplishes the objective set out above with anintervertebral implant, comprising a three-dimensional body having anupper side and an under side which are suitable for abutting the endplates of two adjacent vertebral bodies. The three-dimensional bodyfurther includes a left side surface and a right side surface, a frontsurface and a rear surface, a horizontal middle plane between the upperside and the under side, and a vertical middle plane extending from thefront surface to the rear surface. The three-dimensional body furthercomprising a plurality of boreholes, having openings at least at or nearthe front surface, passing there through and being suitable foraccommodating longitudinal fixation elements. The intervertebral implantfurther including a front plate displaceably disposed as an insert withthe front side of the three-dimensional body, where the front plateincludes a plurality of boreholes having openings and in which thelongitudinal fixation elements can be anchored, and whose openingsoverlap with the openings of the boreholes of the three-dimensionalbody. The intervertebral implant has a securing plate fastenedsubstantially parallel to the front plate in such a manner that theboreholes of the front plate are covered at least partly by the securingplate. An advantage achieved by the present invention, arisesessentially from the solid connection between the intervertebral implantand the longitudinal fixation elements, used to fasten it.

Compared to the two-part implants of the state of the art, for which afront plate is implanted in a separate step, the present invention hasthe advantage that the implantation of the intervertebral implant may becarried out in one step and, with that, can be carried out more easilyand more quickly. A further advantage is that the intervertebral implantis fixed as frontally as possible at the body of the vertebra. That is,at a place where good bone material usually is present. The result is ananterior movement limitation without a greater risk to the surroundingstructures. The load is still absorbed under compression by theintervertebral implant and not by the front plate or the fixation screws(longitudinal fixation elements).

A method for implanting an intervertebral implant of the presentinvention between two adjacent vertebral bodies includes introducing theintervertebral implant, having a three-dimensional body, a front plate,and one or more boreholes, between two adjacent vertebral bodies,attaching longitudinal fixation elements with heads through theboreholes into the vertebral bodies, and attaching a securing plate bymeans of a fastening agent over the heads of the longitudinal fixationelements to the front plate, such that the heads of the longitudinalfixation elements are captured between the front plate and the securingplate wherein the longitudinal fixation elements are secured againstbeing shifted relative to the intervertebral implant.

Other objectives and advantages in addition to those discussed abovewill become apparent to those skilled in the art during the course ofthe description of a preferred embodiment of the invention whichfollows. In the description, reference is made to accompanying drawings,which form a part thereof, and which illustrate an example of theinvention. Such example, however, is not exhaustive of the variousembodiments of the invention, and therefore, reference is made to theclaims that follow the description for determining the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded drawing of the intervertebral implant,

FIG. 2 shows a longitudinal fixation element in the form of a screw,

FIG. 3 shows an elevation of the intervertebral implant of FIG. 1,

FIG. 4 shows a side view of the intervertebral implant of FIG. 1,

FIG. 5 shows a three-dimensional detailed representation of the body ofthe intervertebral implant, which shows the connecting elements to thefront plate of FIG. 6,

FIG. 6 shows a three-dimensional detailed representation of the frontplate of the intervertebral implant and the connecting elements to thebody of FIG. 5 and

FIG. 7 shows a completely installed intervertebral implant with frontplate and securing plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The intervertebral implant, shown in FIG. 1-7, includes athree-dimensional body 10 in the form of a cage with an upper side 1 andan underside 2, which are suitable for abutting the end plates of twoadjacent vertebral bodies, a left side surface 3 and a right sidesurface 4, a front surface 5 and a back surface 6, a horizontal middleplane 7 located between the upper side 1 and the underside 2, a verticalmiddle plane 12 extending from the front surface 5 to the rear surface 6and four boreholes 9 a, which pass through the body 10 and are suitablefor accommodating longitudinal fixation elements 20. The body 10 may beconstructed as a hollow body, the mantle surfaces of which are providedwith perforations 19. The upper side 1 and/or under side 2 of theintervertebral implant may preferably be convex in shape, not planar. Aconvex shape to the upper side 1 and the underside 2 allows for animproved fit with the end plates of the adjacent vertebral bodies by theintervertebral implant. Further, the side surfaces 1-6 of theintervertebral implant may be essentially convex, as well.

As shown in FIG. 7, the upper side 1 and the underside 2 of thethree-dimensional body 10 are provided with structuring in the form ofteeth 30.

At the front surface of the three-dimensional body 10, a front plate 8may be mounted, which is disposed perpendicular to the horizontalcentral plane of the intervertebral implant and through which fourboreholes 9 pass and in which the longitudinal fixation elements 20 canbe anchored. The front plate 8, as shown in FIGS. 5 and 6, isconstructed as an insert for the three-dimensional body 10. Thethree-dimensional body 10 has a semicircular groove 27 extendingparallel to the vertical middle plane 12 at the transitions of the leftside surface 3 and the right side surface 4 (FIG. 5) to the frontsurface 5. Correspondingly, the front plate 3 has right and left (FIG.6) similarly extending and similarly dimensioned, semicircular rail 28.As a result, the front plate can be pushed and positioned easily withits two lateral rails 28 into the corresponding grooves 27 of the body10 during the production of the intervertebral implant.

In one embodiment at least one of the boreholes 9 in the front plate isconstructed so that a longitudinal fixation element 20, accommodatedtherein, can be connected rigidly with the front plate. A rigidconnection may be accomplished, for example, owing to the fact that atleast one of the boreholes 9 of the front plate 8 has an internalthread. A corresponding longitudinal fixation element 20, bone screw,with a threaded end can then be screwed together rigidly with theimplant. In an alternative embodiment, the four boreholes 9 in the frontplate may have an internal thread 11, so that longitudinal fixationelements 20 can be connected rigidly with the front plate 8.

As discussed, the front plate 8 may be disposed, preferably verticallyto the horizontal central plane, so that it can be displaced verticallywith respect to the three-dimensional body 10. By these means, “stressshielding” (protection and neutralization of mechanical stresses) isattained, which permits the end plates to be adapted to theintervertebral implant during the healing process.

The intervertebral implant may have a securing plate 18, which can befastened by means of a screw connection parallel to the front plate 8 atthe front plate 8 in such a manner that the boreholes 9 of the frontplate 3 are partly covered by the securing plate 18. The securing plate18 may have a central borehole 17 provided, preferably, with an internalthread. Corresponding thereto, the front plate 8 has a central borehole15 for accommodating fastening means 16. Preferably, the centralborehole 15 has an internal thread 14 for accommodating a fasteningmeans 16 in the form of a screw. The securing plate 18 may also befastened by a bayonet catch or a click catch. By fastening the securingplate 18 to the front plate 8, the heads 21 of the longitudinal fixationelements 20 (discussed later) are contacted by the securing plate 18, sothat they are secured against being ejected or screwed out.

Preferably, the boreholes 9 a of the three-dimensional body 10 do notpass either through the left side surface 3 or the right side surface 4or completely through the front surface 5. The front surface 5,preferably, is also not crossed by the boreholes 9 a. Further, thehorizontal middle plane 7 is not pierced by the boreholes 9 a. Only theaxes 24 of the longitudinal fixation elements 20, introduced therein,intersect the horizontal middle plane 7 of the body 10. As seen from thefront surface 5, the boreholes of the three-dimensional body 10 and thefront plate diverge. The axes 24 of the boreholes of thethree-dimensional plate 10 and the front plate 8 enclose an angle βranging from 20° to 60°, specifically from 36° to 48°, and morepreferably an angle β of 42° with the horizontal middle plane 7 (FIG. 4)and an angle α ranging from 10° to 45°, specifically from 27° to 33°,and more preferably an angle α of 30° with the vertical middle plane 12(FIG. 3). Thus, better access for introducing the screws is achieved.

In one embodiment, at least one of the boreholes 9 of the front plate 8may taper conically towards the underside 2, so that a bone screw, witha corresponding conical head, can be anchored rigidly therein. Theconical borehole preferably has a conical angle, which is smaller thanthe resulting frictional angle. Advisably, the conicity of the conicalborehole is 1:3.75 to 1:20.00 and preferably 1:5 to 1:15.

In another configuration, at least two of the boreholes 9 of the frontplate 8 extend parallel to each other. This makes insertion of theintervertebral implant easier. In another embodiment, at least two ofthe boreholes 9 of the front plate 8 diverge when viewed from the frontside. By these means, a region of the vertebral body, which has a betterbone quality than does the center of the vertebral body, is reached bythe bone screws.

To improve the anchoring of the bone screw in a plastic body of theintervertebral implant (discussed later), a metal sleeve with aninternal thread (not shown) may be inserted in the boreholes of thefront plate and three-dimensional body. The intervertebral implant mayalso consist only partially of an x-ray transparent plastic and, in theregion of the boreholes consist of a metal, such as titanium or atitanium alloy. Improved guidance and anchoring of the bone screws inthe intervertebral implant may be achieved. Further, the boreholes 9 mayhave a smooth internal wall, into which the threaded head of a metallic,longitudinal fixation element may cut or be molded.

Depending on circumstances, two, three, four or more longitudinalfixation elements may be connected rigidly with the intervertebralimplant. Preferably, at least one fixation element should pierce theupper side and at least one fixation element the underside of theintervertebral implant. The longitudinal fixation elements 20 may haveeither a smooth head, so that there will not be a rigid connection withthe implant or a threaded, conical or expendable end, so that there willbe a rigid connection with the implant. In both cases, however, thelongitudinal fixation elements 20 are secured by the securing plateagainst rotating out, being ejected out or falling out at a later time.

The longitudinal fixation elements 20 are preferably constructed as bonescrews. As shown in FIG. 2, the longitudinal fixation elements 20,introduced into the boreholes 9, have a head 21, a tip 22, a shaft 23and an axis 24. The head 21 may preferably be provided with an externalthread 25, which corresponds to the internal thread 11 of the borehole9, so that the heads 21 can be anchored in the boreholes 9 in a rigidmanner. The shaft 23 may be provided with a thread 26, which isself-drilling and self-cutting. The load thread angle of the thread 26has a range of between 11° to 14°, preferably between 12° and 13°, andmore preferably a load thread angle of 12.5°. The pitch angle of thethread may have a range of between 6° and 10°, preferably between 7° and9°, and more preferably have a pitch angle of 8°. The special pitchangle produces a self-retardation in the thread, thus ensuring that thebone screw will not automatically become loose.

In the case of a second, possibly rigid type of connection, alongitudinal fixation element 20, bone screw, may preferably be used,the head of which tapers conically towards the shaft, the conicity ofthe head corresponding to the conicity of the borehole of theintervertebral implant. The longitudinal fixation elements may also beconstructed as threadless cylindrical pins, which are provided with adrilling tip, preferably in the form of a trocar. A further variationconsist therein that the longitudinal fixation elements are constructedas spiral springs. Finally, the longitudinal fixation elements may alsobe constructed as single-vaned or multi-vaned spiral blades.

As shown in FIG. 7, two longitudinal fixation elements 20 pierce theupper side 1 and two longitudinal fixation elements 20 pierce theunderside 2 of the body 10, thereby anchoring the intervertebral implantto the adjacent vertebral bodies.

The intervertebral implant may be produced from any material which iscompatible with the body. Preferably, the three-dimensional body 10 mayconsist of a body-compatible plastic which has not been reinforced andwhich may be transparent to x-rays. The advantage over fiber-reinforcedplastics, which are already known in implant technology, is that noreinforcing fibers are exposed. Such exposure may be disadvantageousclinically. In such a three-dimensional body 10 constructed of a plasticthat has not been reinforced, the use bone screws may be preferable. Asdiscussed previously, the external thread of the bone screw(s) may havea load thread angle range of 11° to 14°, and preferably between 12° to13°. A comparatively slight inclination of the load flank brings about ahigh clamping force. As a result, radial expansion and the danger offorming cracks in the plastic are reduced. Furthermore, the externalthread of the bone screw(s) may preferably have a pitch angle between 6°and 10° and preferably between 7° and 9°.

The front plate 8 may be made from materials different than thethree-dimensional body 10. The front plate 8 is preferably made from ametallic material. Titanium or titanium alloys are particularly suitableas metallic materials. The complete tension chord arrangement (frontplate and screws) may also be made from implant steel or highly alloyedmetallic materials, such as CoCrMo or CoCrMoC. The advantage of titaniumlies in that there is good tissue compatibility and the good ingrowingbehavior of bones. The advantage of highly alloyed metallic materialslies in their high-strength values, which permit filigree constructions.

A brief description of a surgical procedure follows in order to explainthe invention further.

The intervertebral implant, in the form of a three-dimensional body 10,is introduced between two adjacent vertebral bodies by means of asuitable instrument. Longitudinal fixation elements 20, in the form ofbone screws, securing the three-dimensional body 10 are screwed/insertedby means of a suitable aiming device through the boreholes 9 of thefront plate 8 into the vertebral bodies. The front plate 8 may bedisplaced vertically with respect to the three-dimensional body 10, suchthat the openings of the boreholes 9 a of the three-dimensional plate 10and the boreholes 9 of the front plate 8 overlap, to obtain stressshielding. The securing plate 18 is fastened by means of the fasteningagent 16 in the form of a screw over the heads 21 of the longitudinalfixation elements 20 at the front plate 8, so that the heads 21 of thelongitudinal fixation elements 20 and, with that, the screws themselves,are captured between the front plate 8 and the securing plate 18 andsecured against being shifted relative to the three-dimensional body 10(for example, by falling out or by turning out). The fastening agent 16,in the form of a screw, preferably is provided with a thread, which isdistinguished by a large self-retardation.

The invention claimed is:
 1. An intervertebral implant for implantationbetween two adjacent vertebral bodies, the implant comprising: aplurality of bone fixation elements each including a head and a shaft,the shaft having a thread for engaging one of the adjacent vertebra; athree dimensional body having a left side surface, a right side surface,a front surface, a back surface, an upper side and an underside, theupper side and the underside being sized and configured to contact theone of the two adjacent vertebral bodies, respectively, the threedimensional body further including at least one partial borehole incommunication with the front surface and the upper side of the body andat least one partial borehole in communication with the front surfaceand the underside of the body; a plate having an upper surface and alower surface, the plate contacting the front surface of the threedimensional body, the plate including a plurality of boreholes, at leastone of the plurality of boreholes formed in the plate aligning with theat least one partial borehole in communication with the upper side ofthe body and at least one of the plurality of boreholes formed in theplate aligning with the at least one partial borehole in communicationwith the underside of the body when the three dimensional body iscoupled to the plate so that one of the plurality of bone fixationelements is insertable through one of the plurality of boreholes formedin the plate, through one of the plurality of partial borehole formed inthe three dimensional body and into one of the adjacent vertebralbodies, respectively, wherein the three dimensional body furtherincludes a first height as defined by the upper side and the undersideand the plate includes a second height as defined by the upper and lowersurfaces of the plate, the first height being substantially equal to thesecond height so that the three dimensional body and the plate arecontained between the adjacent vertebral bodies when the implant isinserted between the adjacent vertebral bodies; wherein the plate ismade from a biocompatible metallic material and the three dimensionalbody is made from a biocompatible non-metallic material.
 2. Theintervertebral implant of claim 1, further comprising: a semicirculargroove extending parallel to a vertical middle plane at transitions ofthe left and right side surfaces to the front surface.
 3. Theintervertebral implant of claim 1, wherein the three dimensional body ismade from a biocompatible plastic.
 4. The intervertebral implant ofclaim 1, wherein the plurality of bone fixation elements are bone screwsand the plurality of boreholes formed in the plate include internalthreads, at least a portion of the heads of the bone screws beingexternally threaded for engaging the internal threads of the pluralityof boreholes to anchor the plurality of bone screws to the plurality ofboreholes in a rigid manner.
 5. The intervertebral implant of claim 1,wherein the three dimensional body includes at least one through holeextending from the upper side to the underside.
 6. The intervertebralimplant of claim 1, wherein the body includes a horizontal middle planeand a vertical middle plane, the plurality of partial borehole formed inthe body comprise four body partial borehole, the plurality of boreholesformed in the plate comprise four plate boreholes and the plurality ofbone fixation elements comprise four bone fixation elements, two of thebone fixation elements passing through the upper side of the body andtwo of the bone fixation elements passing through the underside of thebody, the four partial borehole formed in the body and the fourboreholes formed in the plate being arranged such that the heads of thefour bone fixation elements are located between the upper side and anunderside in an assembled configuration.
 7. The intervertebral implantof claim 6, wherein the two of bone fixation elements passing throughthe upper side diverge with respect to one another and the two bonefixation elements passing through the underside diverge with respect toone another.
 8. The intervertebral implant of claim 1, wherein the platemay be pushed and placed easily with respect to the front surface of thebody.
 9. The intervertebral implant of claim 1, further comprising: asecuring plate fastenable to the plate, the securing plate at leastpartially covering each of the plurality of boreholes formed in theplate so that, once implanted, the head of the bone fixation elementsare located between the plate and the securing plate.
 10. Theintervertebral implant of claim 9, wherein the plate includes aninternally threaded central borehole for receiving a central screw forfastening the securing plate to the plate.
 11. The intervertebralimplant of claim 1, wherein the body is convex in shape.
 12. Theintervertebral implant of claim 1, wherein the body includes ahorizontal middle plane and a vertical middle plane, the plurality ofpartial borehole formed in the body and the plurality of boreholesformed in the plate define borehole axes, the borehole axes defining afirst angle β relative to the horizontal middle plane and a second angleα relative to the vertical middle plane, the first angle ranging fromtwenty to sixty degrees (20-60°) and the second angle ranging from tento forty-five degrees (10-45).
 13. The intervertebral implant of claim1, further comprising: a securing mechanism operatively engaging theplate, the securing mechanism at least partially covering each of theplurality of boreholes formed in the plate to inhibit the bone fixationelements from falling out.